In recent years, high-performance engines such as four-stroke engines and multi-cylinder engines for use with personal watercraft have been developed for the purpose of, for example, environmental friendliness. As such an engine, for example, an engine including plug caps with ignition coils is disclosed in Japanese Patent No. 3363747.
The plug caps with ignition coils have a structure in which each ignition coil for applying a high voltage to an ignition plug in response to an ignition command from a controller such as an ECU (electric control unit) is integral with the corresponding plug cap. In fields of automobiles and motorcycles, it has been demonstrated that the plug caps with ignition coils can achieve high power efficiency and correct ignition and can contribute to space saving.
When the plug caps with ignition coils are incorporated into the personal watercraft, the following problem arises.
For example, the personal watercraft has a construction in which the engine is disposed in an internal space of a body, namely, an engine room formed of a fiber-reinforced plastic (FRP), and thus has a substantially closed structure to inhibit water entry from the outside. The plug caps with ignition coils cannot radiate heat sufficiently by natural cooling, because the ignition coils are tightly contained in a limited space. In addition, temperature in the internal space of the body tends to increase because of its closed structure. This may further increase the temperature of the plug caps with ignition coils.